Merge pull request #2 from lucab/ups/example-sized

examples: add a size-sealing example

Author: Luca Bruno

README.md

@@ -38,6 +38,8 @@ fn new_sized_memfd() -> Result<memfd::Memfd> {
 }
 ```
 
+Some more examples are available under [examples](examples).
+
 ## License
 
 Licensed under either of

examples/sized.rs

@@ -0,0 +1,49 @@
+//! A simple example showing how to crate size-sealed memfd.
+//!
+//! It creates a new memfd and seals to on a fixed 1K size.
+//!
+//! This is an example ONLY: do NOT panic/unwrap/assert
+//! in production code!
+
+extern crate memfd;
+
+use std::io::{Seek, SeekFrom, Write};
+
+fn main() {
+    // Create a sealable memfd.
+    let opts = memfd::MemfdOptions::default().allow_sealing(true);
+    let mfd = opts.create("sized-1K").unwrap();
+
+    // Resize to 1024B.
+    mfd.as_file().set_len(1024).unwrap();
+
+    // Add seals to prevent further resizing.
+    let mut seals = memfd::SealsHashSet::new();
+    seals.insert(memfd::FileSeal::SealShrink);
+    seals.insert(memfd::FileSeal::SealGrow);
+    mfd.add_seals(&seals).unwrap();
+
+    // Prevent further sealing changes.
+    mfd.add_seal(memfd::FileSeal::SealSeal).unwrap();
+
+    // Write 1K of data, allowed by size seals.
+    let data_1k = vec![0x00; 1024];
+    let r = mfd.as_file().write_all(&data_1k);
+    assert!(r.is_ok());
+    mfd.as_file().seek(SeekFrom::Start(0)).unwrap();
+
+    // Write 2K of data, now allowed by size seals.
+    let data_2k = vec![0x11; 2048];
+    let r = mfd.as_file().write_all(&data_2k);
+    assert!(r.is_err());
+    mfd.as_file().seek(SeekFrom::Start(0)).unwrap();
+
+    // Try to resize to 2048B, not allowed by size seals.
+    let r = mfd.as_file().set_len(2048);
+    assert!(r.is_err());
+
+    // Overwrite 1K of data, allowed by size seals.
+    let data_1k = vec![0x22; 1024];
+    let r = mfd.as_file().write_all(&data_1k);
+    assert!(r.is_ok());
+}